1. Field of the Invention
This invention relates to optical transmitters. In particular, the invention relates to local control of optical transmitters.
2. Description of Related Art
Existing burst mode laser driver circuits use linear, wide dynamic range transimpedance amplifiers to monitor the optical signal of the laser. This transimpedance amplifier is used to either drive a precision (e.g., 10-bit) analog-to-digital converter or independent comparator circuits to determine the optical levels of the transmitted signal. Existing solutions are implemented with several integrated circuits.
One prior art solution is to use a microprocessor to control the output current sources that drive the semiconductor laser. A photodiode measures the optical radiation emitted by the laser and produces current proportional to the intensity of the optical radiation. This photodiode current is then converted to an analog voltage by a transimpedance amplifier. An analog-to-digital converter (ADC) converts this analog voltage into a digital data representing the photodiode current. The microprocessor reads this digital data and compares the measured photodiode current with the desired values. Based on the result of this comparison, the microprocessor calculates an appropriate control value. The microprocessor then outputs the control value to the current sources to set the driving currents to match the desired values. The process is continuously repeated to keep the output current within the desired operating range. The disadvantages of this technique include the hardware complexity (e.g., the ADC and the microprocessor) and the difficulty to maintain the precise timing of the control.
Another prior art technique uses peak detectors, analog comparators and counters to individually control the current sources. Two branches of circuit are used. The first branch includes a negative peak detector, a first analog comparator, and a counter to control the first current source. The second branch includes a positive peak detector, a second analog comparator, and a second counter to control the second current source. Each of the analog comparators is compared with a suitable threshold value and produces an error value to control the amount and direction of the control quantity by incrementing or decrementing the corresponding counter. The two counters are clocked by two separate clock signals. Although this technique eliminates the ADC and the microprocessor, it still requires two peak detectors and two comparators. In addition, an additional clock generator circuit is required to generate clock signals to the counters.
The disadvantages of the prior art techniques include the hardware complexity which makes it difficult to implement the circuit on a single integrated circuit. In addition, the prior art techniques do not provide precise timing control. This may become a problem when burst data communications are involved because of the short duty cycle of the burst.
Therefore there is a need in the technology to provide an efficient method and apparatus to control the optical transmitter with less hardware and more precise timing.